The Skin Detector System of the LZ Dark Matter Experiment
This paper presents the design, calibration, and performance of the Skin Detector component of the LUX-ZEPLIN (LZ) dark matter experiment. The Skin Detector consists of approximately 2 tonnes of liquid xenon instrumented with 131 photomultiplier tubes, surrounding the central time-projection chamber and functions as an active veto system for background rejection. Energy calibration with monoenergetic gamma sources established a position-dependent energy scale and determined prompt and delayed veto thresholds. Radon decays in the Skin can be identified with less than 15~mm spatial resolution, revealing stable convective flow patterns with measured velocities ranging from 2-4 mm/s that persisted throughout extended data-taking periods. The achieved Skin veto threshold meets or exceeds the design goal of 100~keV throughout the Skin volume. The prompt veto efficiency for gamma rays emitted following electron capture in $^{127}$Xe is found to be ($88 \pm 2$)\%, consistent with the simulated value of ($90.4 \pm 0.6$)\%. M-shell EC decays are used to validate the LZ detector sensitivity for low-mass dark matter during the first science run.